For me, it's a bigger challenge, it's much harder to do and much more rewarding to do well, then just to think up stuff of your own, hit or miss, because you've got to see to it that you don't torpedo any of his punch lines.
Boys are just boys after all, but sometimes girls really seem to be the turn of a pale wrist, or the sudden jut of a hip, or a clutch of very dark hair falling across a freckled forehead. I'm not saying that's what they really are. I'm just saying sometimes it seems that way, and that those details (a thigh mole, a full face flush, a scar the precise shape and size of a cashew nut) are so many hooks waiting to land you.